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        Using the DB Java Collections API
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        Using the DB Java Collections API
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="UsingCollectionsAPI"></a>
        Using the DB Java Collections API
    </h2>
          </div>
        </div>
        <div></div>
      </div>
      <p>
        An 
        
        <a href="../../java/com/sleepycat/db/Environment.html" target="_top">Environment</a>
        
        manages the resources for one or more data stores. A
        
        <a href="../../java/com/sleepycat/db/Database.html" target="_top">Database</a>
        
        object
        represents a single database and is created via a method on the
        environment object. 
        
        <a href="../../java/com/sleepycat/db/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
        
        objects represent an index associated with a primary database.
        
        <span>
            An access method must be chosen for each database and secondary
            database.
        </span>

        Primary and secondary databases are then used to create stored
        collection objects, as described in
        <a href="UsingStoredCollections.html">
        Using Stored Collections
    </a>.
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="UsingTransactions"></a>
            Using Transactions
        </h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
        Once you have an environment, one or more databases, and one or
        more stored collections, you are ready to access (read and write)
        stored data. For a transactional environment, a transaction must be
        started before accessing data, and must be committed or aborted
        after access is complete. The DB Java Collections API provides several
        ways of managing transactions.
    </p>
        <p>
        The recommended technique is to use the 
        <a href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
        
        class along with your own implementation of the 
        <a href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
        
        interface. 
        <a href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
        
        will call your 
        <a href="../../java/com/sleepycat/collections/TransactionWorker.html" target="_top">TransactionWorker</a>
        
        implementation class to perform the data access or work of the
        transaction. This technique has the following benefits:
    </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                Transaction exceptions will be handled transparently and
                retries will be performed when deadlocks are detected.
            </p>
            </li>
            <li>
              <p>
                The transaction will automatically be committed if your
                <a href="../../java/com/sleepycat/collections/TransactionWorker.html#doWork()" target="_top">TransactionWorker.doWork()</a>
                
                method returns normally, or will be
                aborted if <tt class="methodname">doWork()</tt> throws an exception.
            </p>
            </li>
            <li>
              <p>
                <tt class="classname">TransactionRunner</tt> can be used for non-transactional
                environments as well, allowing you to write your application
                independently of the environment.
            </p>
            </li>
          </ul>
        </div>
        <p>
        If you don't want to use 
          <a href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>,
        the alternative is to use the 
        <a href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>
        
        class.
    </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>
                Obtain a CurrentTransaction instance by calling the
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#getInstance(com.sleepycat.db.Environment)" target="_top">CurrentTransaction.getInstance</a>
                
                method. The instance returned
                can be used by all threads in a program.
            </p>
            </li>
            <li>
              <p>
                Use
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#beginTransaction(com.sleepycat.db.TransactionConfig)" target="_top">CurrentTransaction.beginTransaction()</a>,
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#commitTransaction()" target="_top">CurrentTransaction.commitTransaction()</a>
                
                and
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#abortTransaction()" target="_top">CurrentTransaction.abortTransaction()</a>
                
                to directly begin, commit and abort transactions.
            </p>
            </li>
          </ol>
        </div>
        <p>
        If you choose to use CurrentTransaction directly you must handle
        the 
        
        <a href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>
        
        exception and perform retries yourself. Also note that
        CurrentTransaction may only be used in a transactional
        environment.
    </p>
        <p>
        The DB Java Collections API supports nested transactions. If
            <a href="../../java/com/sleepycat/collections/TransactionRunner.html#run(com.sleepycat.collections.TransactionWorker)" target="_top">TransactionRunner.run(com.sleepycat.collections.TransactionWorker)</a>
            
            or 
            <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#beginTransaction(com.sleepycat.db.TransactionConfig)" target="_top">CurrentTransaction.beginTransaction()</a>
            ,
            is called while another transaction is active, a child transaction
            is created. When 
            <a href="../../java/com/sleepycat/collections/TransactionRunner.html#run(com.sleepycat.collections.TransactionWorker)" target="_top">TransactionRunner.run(com.sleepycat.collections.TransactionWorker)</a>
            
            returns, or when
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#commitTransaction()" target="_top">CurrentTransaction.commitTransaction()</a>
                
            or
                <a href="../../java/com/sleepycat/collections/CurrentTransaction.html#abortTransaction()" target="_top">CurrentTransaction.abortTransaction()</a>
                
            is called, the parent transaction becomes active again. Note that
            because only one transaction is active per-thread, it is impossible
            to accidentally use a parent transaction while a child transaction
            is active.
    </p>
        <p>
        The DB Java Collections API supports transaction auto-commit.
        If no transaction is active and a write operation is requested for
        a transactional database, auto-commit is used automatically.
    </p>
        <p>
        The DB Java Collections API also supports transaction
        dirty-read via the 
        <a href="../../java/com/sleepycat/collections/StoredCollections.html" target="_top">StoredCollections</a>
        
        class. When dirty-read is enabled for a collection, data will be
        read that has been modified by another transaction but not
        committed. Using dirty-read can improve concurrency since reading
        will not wait for other transactions to complete. For a
        non-transactional container, dirty-read has no effect. See 
        <a href="../../java/com/sleepycat/collections/StoredCollections.html" target="_top">StoredCollections</a>
        
        for how to create a dirty-read collection.
    </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="TransactionRollback"></a>
            Transaction Rollback
        </h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
        When a transaction is aborted (or rolled back) the application
        is responsible for discarding references to any data objects that
        were modified during the transaction. Since the DB Java Collections API
        treats data by value, not by reference, neither the data
        objects nor the DB Java Collections API objects contain status
        information indicating whether the data objects are 1- in sync with
        the database, 2- dirty (contain changes that have not been written
        to the database), 3- stale (were read previously but have become
        out of sync with changes made to the database), or 4- contain
        changes that cannot be committed because of an aborted
        transaction.
    </p>
        <p>
        For example, a given data object will reflect the current state
        of the database after reading it within a transaction. If the
        object is then modified it will be out of sync with the database.
        When the modified object is written to the database it will then be
        in sync again. But if the transaction is aborted the object will
        then be out of sync with the database. References to objects for aborted
        transactions
        should no longer be used. When these objects are needed later they
        should be read fresh from the database.
    </p>
        <p>
        When an existing stored object is to be updated, special care
        should be taken to read the data, then modify it, and then write it
        to the database, all within a single transaction. If a stale data
        object (an object that was read previously but has since been
        changed in the database) is modified and then written to the
        database, database changes may be overwritten unintentionally.
    </p>
        <p>
        When an application enforces rules about concurrent access to
        specific data objects or all data objects, the rules described here
        can be relaxed. For example, if the application knows that a
        certain object is only modified in one place, it may be able to
        reliably keep a current copy of that object. In that case, it is
        not necessary to reread the object before updating it. That said,
        if arbitrary concurrent access is to be supported, the safest
        approach is to always read data before modifying it within a single
        transaction.
    </p>
        <p>
        Similar concerns apply to using data that may have become stale.
        If the application depends on current data, it should be read fresh
        from the database just before it is used.
    </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="SelectingAccessMethods"></a>Selecting Access Methods</h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
        For each data store and secondary index, you must choose from one of the
        access methods in the table below.

        The access method determines not only whether sorted keys or duplicate
        keys are supported, but also what types of collection views may be used
        and what restrictions are imposed on the collection views.
    </p>
        <div class="informaltable">
          <table border="1" width="80%">
            <colgroup>
              <col />
              <col />
              <col />
              <col />
              <col />
              <col />
            </colgroup>
            <thead>
              <tr>
                <th>Access Method</th>
                <th>Ordered</th>
                <th>Duplicates</th>
                <th>Record Numbers</th>
                <th>Database Type</th>
                <th><tt class="classname">DatabaseConfig</tt> Method</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>
                        BTREE-UNIQUE
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        No
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#BTREE" target="_top">BTREE</a>
                </td>
                <td>
                        None
                    </td>
              </tr>
              <tr>
                <td>
                        BTREE-DUP
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        Yes, Unsorted
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#BTREE" target="_top">BTREE</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setUnsortedDuplicates(boolean)" target="_top">setUnsortedDuplicates</a>
                </td>
              </tr>
              <tr>
                <td>
                        BTREE-DUPSORT
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        Yes, Sorted
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#BTREE" target="_top">BTREE</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setSortedDuplicates(boolean)" target="_top">setSortedDuplicates</a>
                </td>
              </tr>
              <tr>
                <td>
                        BTREE-RECNUM
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Renumbered
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#BTREE" target="_top">BTREE</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setBtreeRecordNumbers(boolean)" target="_top">setBtreeRecordNumbers</a>
                </td>
              </tr>
              <tr>
                <td>
                        HASH-UNIQUE
                    </td>
                <td>
                        No
                    </td>
                <td>
                        No
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#HASH" target="_top">HASH</a>
                </td>
                <td>
                        None
                    </td>
              </tr>
              <tr>
                <td>
                        HASH-DUP
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Unsorted
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#HASH" target="_top">HASH</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setUnsortedDuplicates(boolean)" target="_top">setUnsortedDuplicates</a>
                </td>
              </tr>
              <tr>
                <td>
                        HASH-DUPSORT
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Sorted
                    </td>
                <td>
                        No
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#HASH" target="_top">HASH</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setSortedDuplicates(boolean)" target="_top">setSortedDuplicates</a>
                </td>
              </tr>
              <tr>
                <td>
                        QUEUE
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Fixed
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#QUEUE" target="_top">QUEUE</a>
                </td>
                <td>
                        None
                    </td>
              </tr>
              <tr>
                <td>
                        RECNO
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Fixed
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#RECNO" target="_top">RECNO</a>
                </td>
                <td>
                        None
                    </td>
              </tr>
              <tr>
                <td>
                        RECNO-RENUMBER
                    </td>
                <td>
                        Yes
                    </td>
                <td>
                        No
                    </td>
                <td>
                        Yes, Renumbered
                    </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseType.html#RECNO" target="_top">RECNO</a>
                </td>
                <td>
                  <a href="../../java/com/sleepycat/db/DatabaseConfig.html#setRenumbering(boolean)" target="_top">setRenumbering</a>
                </td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
        Please see
            <span class="html"><a href="../../ref/am_conf/intro.html" target="_top">Available Access Methods</a> in</span>
            the <i class="citetitle">Berkeley DB Programmer's Reference Guide</i>
        for more information on access method configuration.
    </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="AccessMethodRestrictions"></a>
            Access Method Restrictions
        </h3>
            </div>
          </div>
          <div></div>
        </div>
        <p>
        The restrictions imposed by the access method on the database
        model are:
    </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                If keys are ordered then data may be enumerated in key order and
                key ranges may be used to form subsets of a data store. The 
                <tt class="classname">SortedMap</tt> and <tt class="classname">SortedSet</tt>
                interfaces are supported for collections with ordered keys.
            </p>
            </li>
            <li>
              <p>
                If duplicates are allowed then more than one value may be
                associated with the same key. This means that the data store cannot
                be strictly considered a map — it is really a multi-map. See
                <a href="UsingStoredCollections.html">
        Using Stored Collections
    </a>
                for implications on the use of the collection interfaces.
            </p>
            </li>
            <li>
              <p>
                If duplicate keys are allowed for a data store then the data
                store may not have secondary indices.
            </p>
            </li>
            <li>
              <p>
                For secondary indices with duplicates, the duplicates must be
                sorted. This restriction is imposed by the DB Java Collections API.
            </p>
            </li>
            <li>
              <p>
                With sorted duplicates, all values for the same key must be
                distinct.
            </p>
            </li>
            <li>
              <p>
                If duplicates are unsorted, then values for the same key must be
                distinct.
            </p>
            </li>
            <li>
              <p>
               If record number keys are used, the the number of records is
               limited to the maximum value of an unsigned 32-bit integer. 
            </p>
            </li>
            <li>
              <p>
                If record number keys are renumbered, then standard List
                add/remove behavior is supported but concurrency/performance is
                reduced.
            </p>
            </li>
          </ul>
        </div>
        <p>
        See 
        <a href="UsingStoredCollections.html">
        Using Stored Collections
    </a>
        for more information on how access methods impact the use of stored
        collections.
    </p>
      </div>
    </div>
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        API Notes and Details
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